The manufacturing of integrated circuits aims for continuously decreasing feature sizes of the fabricated components. Decreasing of feature sizes of the fabricated components can be achieved by printing elements using a lithographic patterning process with higher resolution capabilities.
The pattern being projected is provided on a photo mask. The photo mask is illuminated by a light source having a wavelength ranging from ultraviolet (UV) light to deep-UV light in modern applications. The part of the light that is not blocked or attenuated by the photo mask is projected onto the resist film layer on the surface of a semiconductor wafer using a lithographic projection apparatus. The lithographic projection apparatus comprises a projection lens that usually performs a reduction of the pattern contained on the photo mask, e.g., by a factor of four.
In order to manufacture patterns having line widths in the range of 90 nm or smaller, large efforts have to be undertaken to further enhance the resolution capabilities of a lithographic projection apparatus.
The achievable resolution is determined by several factors. In optical lithography the relationship between the maximal resolution depends on the illumination wavelength and the numerical aperture of the lens of the projection system.
While the illumination wavelength and the numerical aperture are fixed for a given generation of a lithographic projection technology, optimizing the illumination process and implementing so called resolution enhancement techniques (RET) reduces the technology characterising coefficient and thus improves the resolution capabilities of the lithographic projection apparatus.
Currently, there are two concepts known in the art that address the problem of increasing the resolution capabilities. First, off-axis illumination in the projection system of the projection apparatus together with sub-resolution sized assist features is used. Second, the concept of alternating phase shift masks is employed so as to enhance the resolution capabilities of the projection apparatus.
These concepts increase the resolution capabilities in semiconductor manufacturing. However, significant efforts and investments are needed to produce memories having the best possible resolution capabilities.